Chain-Length-Dependent Photophysical Properties of α,ω-Di(4-pyridyl)polyenes: Effects of Solvent Polarity, Hydrogen Bond Formation, Protonation, and N-Alkylation.

In this study, we investigated the solvent effects on the photophysical properties of α,ω-di(4-pyridyl)polyenes 1-5 having 1-5 double bonds. The solution photoproperties depend strongly on conjugation chain length. The absorption maximum (λa) of dipyridylethylene 1 is observed at around 290 nm and only slightly redshifts as the solvent polarity increases, whereas its fluorescence maximum (λf) redshifts from 368 nm in hexane to 403 nm in acetonitrile. Although 1 is a centrosymmetric molecule, its fluorescence energy linearly correlates with the Onsager solvent polarity function f(ε) - f(n2) = (ε - 1)/(2ε + 1)-(n2 - 1)/(2n2 + 1), indicating that the emission originates from an intramolecular charge transfer (CT) excited state. Exceptionally, λf in methanol is largely blue-shifted to 341 nm from those in other aprotic solvents. The fluorescence solvatochromism of longer polyenes 2-5 is much less significant than that of 1. Upon protonation and N-alkylation, both the absorption and fluorescence spectra of all five compounds are red-shifted in methanol. The largest shifts in λa and λf on protonation are observed for pentaene 5 (73 nm) and diene 2 (57 nm), respectively.